Conversion of natural gas to liquid hydrocarbons (“Gas To Liquids” or “GTL” process) is based on a 3 step procedure consisting of: 1) synthesis gas production; 2) synthesis gas conversion by FT synthesis; and 3) upgrading of FT products (wax and naphtha/distillates) to final products such as naphtha, kerosene, diesel or other products, for example lube oil base.
Supported cobalt catalysts are the preferred catalysts for the FT synthesis. The most important properties of a cobalt FT catalyst are the activity, the selectivity usually to C5 and heavier products and the resistance towards deactivation. Known catalysts are typically based on titania, silica or alumina supports and various metals and metal oxides have been shown to be useful as promoters.
A recent series of papers by Iglesia et al. including “Selectivity Control and Catalyst Design in the Fischer-Tropsch Synthesis: Sites, Pellets and Reactors” Advances in Catalysis, Vol. 39, 1993, p. 221-302, has given a description of the reaction network leading to various hydrocarbon products and a methodology to optimize catalyst properties towards the desired heavy hydrocarbons. The maximum C5 + selectivity is obtained by designing catalyst pellets with optimum intraparticle diffusion resistance. This is achieved by increasing intraparticle diffusion resistance to the point where secondary chain building reactions of primary products (alpha-olefins) are maximized without inducing significant diffusion resistance on the reactants (H2, CO) because this will lead to poor selectivity. This principle is shown to be generally applicable on all the supports mentioned above. By plotting different catalysts with different physical properties (particle size, porosity, cobalt loading, cobalt dispersion) a typical “volcano plot” is generated and the maximum C5 + selectivity is found for intermediate values of a parameter “χ” which is a function of the parameters mentioned above and is a measure of the intraparticle diffusion resistance at a given set of reaction conditions.
Definition of χ:χ=R02øθ/rp  (1)where:
R0=Catalyst particle radius (m)
ø=Catalyst porosity
θ=Catalytic site density (sites/m2)
rp=average pore radius (m)
According to Iglesia the optimum value of χ for a typical set of FT reaction conditions (200° C., 20 bar, H2/CO=2.1, 50-60% conversion) is about 500-1000×10−16 m−1, irrespective of the nature of the catalyst support used. From the definition of χ it appears that any of the parameters involved (particle radius, porosity, pore radius or site density) can be varied to achieve the desired value of χ. However, this is somewhat misleading due to the known relationship between specific surface area, pore radius and porosity (or specific pore volume). By introducing these relationships, it will be seen that χ can be described by the particle size, the cobalt loading, the cobalt dispersion and the porosity. Thus, it can be seen that χ is actually independent of pore radius and site density and is determined only by the volumetric transport parameter which is controlled solely by particle size, the cobalt loading, the cobalt dispersion and the porosity.
The following known equations are valid for an ideal cylindrical pore structure:rp=2Vg/Sg  (2)Vg=ø/ρp  (3)ρp=(1−ø)ρs  (4)where
Vg=specific pore volume (cm3/g)
Sg=specific surface area (m2/g)
ρp=particle density (g/cm3)
ρs=material density (g/cm3)
The site density term in (1) (θ=Co sites/m2) can be expressed by:θ=Co sites/m2surface area=XCoDCoA/SgMCo  (5)where
XCo=Total Co concentration in catalyst (gCo/gcat)
DCo=Co dispersion (fraction of total Co exposed)
A=Avogadro number=6.23 1023 atoms/mole
MCo=Co molecular weight=58.9 g/mole
By combining equations (2)-(5) with (1) it can be shown that χ can be written as:χ=R02XCoDCoA(1−ø)ρs/2MCo  (6)
It is apparent from (6) that χ actually is independent of pore radius and only depends on the volumetric density of sites in the free pore volume of the catalyst. It is also clear that due to the second order dependency on particle size, the easiest way of controlling χ is to vary the particle size.
If a cobalt catalyst is to be used in a fixed-bed type reactor it is necessary to use particle sizes of 1 mm or larger in order to avoid unacceptable pressure drop over the reactor. However, the value of χ is then far too high to achieve optimum selectivity, due to high reactant diffusion resistance. This can to a certain extent be addressed by the use of so called eggshell or rim type catalysts where the active cobalt containing phase is located in a relatively thin region in the outer shell of the support. However, in slurry type reactors, it is necessary to employ much smaller particles, typically 10-100 μm. It is then easily seen that it will be extremely difficult to achieve χ values in the desired region. For example, a catalyst with 10 weight % cobalt loading, 5% Co dispersion, 50% porosity and 50 micron particles will have χ=13×1016 m−1.
It should also be kept in mind that the parameters in eq. (6) can generally not be changed independently, i.e. the higher Co loading the more difficult it is to achieve a high dispersion. Moreover, the lower the porosity the more difficult it becomes to use a high cobalt loading. A combination of 20 weight % cobalt loading, 10% Co dispersion and 30% porosity gives a higher volumetric cobalt density than can be seen in any reference known to the applicants. The corresponding value of χ for a 50 μm particle (which is suitable for slurry reactor operation) will then be 75×1016 m−1, which is still far lower than the optimum value taught by Iglesia.
Thus, there is no apparent teaching for preparing high selectivity catalysts for use with small particle sizes, such as are encountered in slurry reactors.